Subjects were studied on their long-term medications in the post-absorptive state and supine position. Right heart catheterization was performed with simultaneous expired gas analysis at rest and during supine exercise at a 20 Watts workload for 5 minutes. After the first exercise phase (before any drug administration) and after return to steady-state baseline hemodynamic values, subjects were randomized. Study drug or placebo was infused for 5 minutes. After a 10 minute observation period, hemodynamic measurements were repeated at rest, followed by repeat supine exercise at a 20 Watts workload for 5 minutes, identical to the study's first phase. Arterial and venous blood samples and hemodynamic and expired gas data were acquired during each stage of the protocol.